The equation of a circle $C$ is $x^2+y^2+8x+15 = 0$. What is its center $(h, k)$ and its radius $r$ ?
To find the equation in standard form, complete the square. $(x^2+8x) + (y^2) = -15$ $(x^2+8x+16) + (y^2) = -15 + 16 + 0$ $(x+4)^{2} + y^2 = 1 = 1^2$ Thus, $(h, k) = (-4, 0)$ and $r = 1$.